Nitrite activates protein kinase A in normoxia to mediate mitochondrial fusion and tolerance to ischaemia/reperfusion

AIMS: Nitrite (NO2(-)), a dietary constituent and nitric oxide (NO) oxidation product, mediates cardioprotection after ischaemia/reperfusion (I/R) in a number of animal models when administered during ischaemia or as a pre-conditioning agent hours to days prior to the ischaemic episode. When present during ischaemia, the reduction of nitrite to bioactive NO by deoxygenated haem proteins accounts for its protective effects. However, the mechanism of nitrite-induced pre-conditioning, a normoxic response which does not appear to require reduction of nitrite to NO, remains unexplored. METHODS AND RESULTS: Using a model of hypoxia/reoxygenation (H/R) in cultured rat H9c2 cardiomyocytes, we demonstrate that a transient (30 min) normoxic nitrite treatment significantly attenuates cell death after a hypoxic episode initiated 1 h later. Mechanistically, this protection depends on the activation of protein kinase A, which phosphorylates and inhibits dynamin-related protein 1, the predominant regulator of mitochondrial fission. This results morphologically, in the promotion of mitochondrial fusion and functionally in the augmentation of mitochondrial membrane potential and superoxide production. We identify AMP kinase (AMPK) as a downstream target of the mitochondrial reactive oxygen species (ROS) generated and show that its oxidation and subsequent phosphorylation are essential for cytoprotection, as scavenging of ROS prevents AMPK activation and inhibits nitrite-mediated protection after H/R. The protein kinase A-dependent protection mediated by nitrite is reproduced in an intact isolated rat heart model of I/R. CONCLUSIONS: These data are the first to demonstrate nitrite-dependent normoxic modulation of both mitochondrial morphology and function and reveal a novel signalling pathway responsible for nitrite-mediated cardioprotection.